The kidney contains several distinct epithelia that, in their aggregate function, are responsible for formation of the urine. We are studying the roles of these epithelia in the regulation of the excretion of water, urea, ammonium, bicarbonate, sodium, potassium, and chloride. The general approach is to dissect the epithelia from the kidney and to study their functions in vitro. Studies in the proximal tubule of the rabbit have demonstrated transport of both ammonium ions and ammonia molecules. Bicarbonate absorption and ammonium secretion have been demonstrated for the first time in the proximal straight tubule of the rat. Studies in the rabbit thick ascending limb have demonstrated active reabsorption of ammonium ions. Studies in rabbit cortical collecting ducts have shown formation of a disequilibrium pH which enhances ammonia secretion. Experiments in the outer medullary collecting duct have demonstrated functional carbonic anhydrase in the lumen of the inner stripe segment, but not in the outer stripe segment. Experiments in the inner medullary collecting duct have shown that vasopressin increases the urea permeability of the terminal part, but not the initial part of the inner medullary collecting duct. The increase urea permeability is mediated by a rise in intracellular cyclic AMP, but vasopressin also increases intracellular calcium. Experiments in the inner medullary collecting duct have shown that atrial natriuretic factor inhibits vasopressin-stimulated water permeability and reduces chloride permeability. Experiments in isolated rabbit papillary surface epithelium have demonstrated a low osmotic water permeability that is not affected by vasopressin.